I have an 18" Opaque that I am going to be using in my upcoming 10.6" wxga build.

Right now it's FL is aprox. 450 mm. I want to get it more down in the 400 - 425 range (as low as I can go without losing image quality).

Question 1: Is this really possible? I've asked this in another thread earlier and I got the impression that if I reduced the spacing between the outer lenses and the middle lens equally, I should achieve the desired results.

Question 2: Will picture quality be affected? If so I may have to find a good compromise between FL and PQ.

Question 3: Does spacer material matter? How hot do you think the triplet gets and should I worry about thermal expansion of my replacement spacers?

Question 4: What is the best way to accurately measure triplet FL?

Thanks for your help. Hopefully this mod can work and I can get my 90" images from 12' away

From what I understand the lens spacing need to be increased no decreased.

I would make the front spacing 2mm longer and the back 4mm but that is just a guess.

Would you mind elaborating... what lead you to this "guess"? Any insight on measuring FL?

Are you sure you are talking about shortening the focal length? If you are wanting to reduce the size of your screen at the 12' throw then you would want to make the focal length longer, not shorter. I only mention this because the one I want to start building will also have a 12' throw and I am looking at a 96" screen using the 22" Buhl opaque lens.

To find the fl just place a 60 watt light bulb on your desk with logo on the top and set the triplet to project an image on the wall. The distance from the middle of the triplet to the top of the lamp is the FL

The second way is just hold it up to the wall and point it at a window.

You say you want as low FL as possible without reducing image quality-- why dont you use the LL standard lens? The projection might be bigger than your 90" at 12ft (no focalcalc here). I'm not sure how flexible you are with the distance from the wall...

I think the key to answer your question is the sentence "I have an 18" Opaque that I am going to be using".

Hence to spend more money on another lense doesn't make sense if you can modify the one you own to do the job. The standard lense only has a focal length of arouns 320mm where he is saying he wants "400 - 425 range" so the standard lense doesn't cut it.